1. Field of the Invention
The present invention relates to a luminous flux branching element and a mask defect inspection apparatus, and in particular, relates to a luminous flux branching element arranged diagonally to incident light to split the incident light from a plane of incidence into a main luminous flux emitted from an emission position on an emission surface and a branched luminous flux emitted from a branch position apart from the emission position and having a smaller quantity of light than that of the main luminous flux and a mask defect inspection apparatus including the luminous flux branching element.
2. Related Art
A luminous flux branching element (optical element) including a luminous flux transmission portion that allows a luminous flux to be incident and a luminous flux reflection portion that reflects all or a portion of the luminous flux incident from the luminous flux transmission portion on a surface of a parallel transparent plate and provided with a luminous flux branching film having a different reflectance has been known as a luminous flux branching element.
For example, a luminous flux branching element provided with a luminous flux transmission portion that allows a luminous flux to be incident and a luminous flux reflection portion that reflects all or a portion of the luminous flux incident on a parallel plate from the luminous flux transmission portion on a surface of the parallel plate and provided with a luminous flux branching film having a different reflectance positioned in each arrival position of the luminous flux repeatedly reflected between the other surface of the parallel plate and the luminous flux reflection portion after being incident from the luminous flux transmission portion on the other surface of the parallel plate is described. (See Published Unexamined Japanese Patent Application No. 7-311302 (JP-A-7-311302), for example.)
A luminous flux branching element described below is also known as another luminous flux branching element. FIGS. 3 and 4 are schematic diagrams showing the luminous flux branching element. A luminous flux branching element 40 shown in FIG. 3 has optical branching (beam splitter, BS) coats 44, 45 whose transmittance is 70% and whose reflectance is 30% deposited on a plane of incidence 42 and a plane of emission 43 of a translucent member 41 formed of a plate made of, for example, optical glass. The luminous flux branching element 40 splits the main luminous flux quantity:branched luminous flux quantity to about 10:1.
A luminous flux branching element 50 shown in FIG. 4 has an AR coat 54 and a mirror coat 55 deposited on a plane of incidence 52 of a translucent member 51 formed of a plate made of, for example, optical glass and an AR coat 56 deposited on a plane of emission 53 in a partial range. In this case, if the incident light is S polarized light, the main luminous flux quantity:branched luminous flux quantity is split to about 10:1.
However, a general luminous flux branching element as shown in JP-A-7-311302 has gradation in the coat in order to make the quantity of light of each branched luminous flux uniform to some extent. The luminous flux branching element is suitable to split into luminous fluxes whose diameter is small, but unevenness of the quantity of light in each branched luminous flux tends to increase and thus, the luminous flux branching element is not suitable to split into luminous fluxes whose diameter is large.
The luminous flux branching element 40 shown in FIG. 3 loses 45% of the total quantity of light as reflected light, resulting in low illumination efficiency. Further, the luminous flux branching element 50 shown in FIG. 4 has unevenness of the boundary between the AR coat 54 and the mirror coat 55 of 0.1 to 1 mm, which makes it impossible to split or difficult to make adjustments when the interval between the main luminous flux and the branched luminous flux is small.
Incidentally, a mask inspection apparatus is known as an enlarging observation apparatus using such a luminous flux branching element. In the mask inspection apparatus, two branched luminous fluxes are used as a luminous flux for mask inspection and a luminous flux for autofocus. In this case, the quantity of light of the luminous flux for autofocus is small, but the luminous flux for autofocus needs to be formed in a region separate from a mask defect detection illumination region and further, a predetermined distance needs to be maintained from the region to the mask defect detection illumination region.
Thus, when the mask defect detection illumination region and the autofocus region are illuminated together, a reflected illumination region needs to be increased, which makes it impossible to use the quantity of light effectively. Therefore, the quantity of light reaching the surface of a sensor decreases, causing a problem of delayed inspection throughput.